It is well known to distribute information, for example, music or motion pictures, on a disc or other portable medium. The medium may be either rented or sold to a member of the public who wishes to listen to or view the content of the medium. Sale prices are higher than rental fees, because a purchaser who retains permanent possession of a copy of the content gains greater benefit than a renter who has the medium with the content for only a short time. In order to prevent users from improperly exploiting the price difference, rental stores take steps to ensure that copies of the medium are returned at the end of the rental period. However, these returns are expensive for the store and inconvenient for the customer.
In order to eliminate the administrative cost and inconvenience of rental returns, it has previously been proposed to provide a storage medium that becomes unusable within a short period after the medium is removed from its packaging or after the medium is first read. For rental of motion pictures, a period of a few hours to a few days is typically appropriate.
Examples of previously-proposed limited life media are described in commonly invented U.S. Pat. No. 6,468,619, and U.S. Patent Application Publication No. 2005/0195728, and in U.S. Pat. No. 6,100,772.
With all of these devices, either reading the storage medium or some step preliminary to such reading, such as removing the storage medium from a container, initiates a process that renders the storage medium unusable. In many of these previously proposed devices, the agent that limits the life of the medium is a liquid, such as a solution of a mild acid or other corrosive agent in water, and the process of rendering the medium unusable is initiated by moving the solution from a storage location within the medium to an active location at which the liquid is in contact with a part of the storage medium that actually carries data.
U.S. Patent Application Publication No. 2005/0195728 proposes an optical disc that self-destructs within a predetermined period after the disc is first read. The disc described in U.S. Patent Application Publication No. 2005/0195728 contains a reservoir of solvent near its center. The process of reading the disc involves rotating the disc at high speed. Centrifugal force from the rotation redistributes the solvent to a location where the solvent destroys part of the data storage layer. The disc of U.S. Patent Application Publication No. 2005/0195728 is well suited to discs of the CD or DVD type, in which the data storage layer is a thin metal foil, susceptible to destruction by mild acids, and in which the inner edge of the data storage layer carries a vital control track. By suitable selection of the strength of the acid, the time within which the disc becomes unusable can be selected in a range from minutes to days.
In the interests of economy and reliability, a simple design of the mechanism for moving the solution to the active location is desirable. The mechanism of U.S. Patent Application Publication No. 2005/0195728 is especially simple. That mechanism has no moving parts, and is operated solely by the liquid flowing under the action of centrifugal force when the disc rotates. However, where a liquid flowing under centrifugal force is used, the fluid properties of the liquid, such as viscosity and surface tension, are significant. It is well within the ordinary skill in the art to formulate suitable liquid agents by adjusting the composition of the liquid and the dimensions of the chambers and passages within which the liquid is contained. However, the correct balance of properties to ensure that the liquid flows sufficiently easily for the limited operating life to be initiated reliably when the disc is first read, without flowing so easily that the operating life is initiated prematurely by jolts or jerks in transport, may require precise formulation.
There is therefore a continuing need for an improved optical disc or other storage medium that more reliably initiates its limited-life function when, and not until, the disc is first played.